The need to define the events of tumor-host interactions in modern immunological terms is apparent. By using autochthonous and transplanted syngeneic tumors in mice, especially murine mammary tumor virus (MuMTV)-induced mammary adenocarcinomas, we will continue our studies on the immune response of the host against such tumors. Our aims will include studies on the specific (and less-specific) cytotoxic T lymphocytes (CTL) induced by in vivo or in vitro immunization, as well as CTL lines. The properties of CTL effectors and of the components which regulate their function will be studied, with special emphasis on regulatory cells of the suppressor type. The main goal of these studies is to define a strategy, based on immunological intervention, which will affect behavior of transplanted mammary tumors and, more importantly, spontaneous mammary tumor development in the high-risk MuMTV-infected C3H/BiUmc (C3H) virgin females. This information may be relevant for planning similar immunological strategies for clinical use, especially those derived from the in vivo effects of CTL lines and of other T-cell lines with regulatory function, or from our models for abrogation of suppressor T cells in vivo.